Field of the Invention
This invention relates to modifications in the reactant outlet manifolds of fuel cell stacks in order to prevent the formation of ice blockages at below freezing temperatures. It particularly relates to modifications in the oxidant outlet manifolds of solid polymer electrolyte fuel cell stacks.
Description of the Related Art
Fuel cells such as solid polymer electrolyte or proton exchange membrane fuel cells electrochemically convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. Solid polymer electrolyte fuel cells generally employ a proton conducting, solid polymer membrane electrolyte between cathode and anode electrodes. A structure comprising a solid polymer membrane electrolyte sandwiched between these two electrodes is known as a membrane electrode assembly (MEA). In a typical fuel cell, flow field plates comprising numerous fluid distribution channels for the reactants are provided on either side of a MEA to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1V, a plurality of cells is usually stacked together in series for commercial applications in order to provide a higher output voltage. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
Along with water, heat is a significant by-product from the electrochemical reactions taking place within the fuel cell. Means for cooling a fuel cell stack is thus generally required. Stacks designed to achieve high power density (e.g. automotive stacks) typically circulate liquid coolant throughout the stack in order to remove heat quickly and efficiently. To accomplish this, coolant flow fields comprising numerous coolant channels are also typically incorporated in the flow field plates of the cells in the stacks. The coolant flow fields may be formed on the electrochemically inactive surfaces of the flow field plates and thus can distribute coolant relatively evenly throughout the cells while keeping the coolant reliably separated from the reactants.
Reactant and coolant manifolds are generally used to provide both reactants and the coolant to and from the individual cells in the stack. A variety of designs can be considered in this regard. For instance, a series of ports or ducts can be provided in various manners at opposing ends of the plates for individual cells such that when the cells are stacked together they form manifolds for these fluids. Further design features that may be required then are passageways to distribute the bulk fluids to and from the various channels in the reactant and coolant flow field channels in the plates. These passageway regions are referred to as the transition regions. The various transition regions can themselves comprise numerous fluid distribution channels, e.g. fuel transition channels in a fuel transition region.
In fuel cell stacks subject to freezing temperatures, accumulations of liquid water can be problematic because, when the water freezes, the ice formed can undesirably block fluid flows and the associated expansion of the solid ice can cause damage to cells in the fuel cell stack. Significant sized accumulations of liquid water which may be subject to freezing are therefore generally avoided, either by preventing accumulation in the first place or alternatively by removing them before they have the opportunity to freeze. For these and other reasons, various designs and techniques are disclosed in the art for managing and controlling water movement within a fuel cell stack.
Despite the advances made to date, there remains a need for better designs and methods to prevent ice blockages from occurring in such fuel cell stacks when subzero temperatures may be encountered. This invention fulfills these needs and provides further related advantages.